\section{Introduction}

%Les arbres ont besoin de se redresser

One of the challenges for trees is to grow up vertically towards the light i.e. grow up in the direction opposed to the gravity field of earth. All along their life, plants make imperceptible movements which allow them to remain straight right. They are submitted to the double challenge of the gravity and the wind \citep{Almeras2009b} and they can remain straight right only by an active control. Trees have to set up a strong enough structure to resist to the buckling under their own weight but also to recover after a disturbance \citep{Jaouen2007}. So, no tree is motionless. They spend their lives to correct their posture as they grow up. \\

%Le gravitropisme est le moteur du redressement et joue un rôle dans les stratégies de recherche de lumière

Trees, like the other plants, feel and react to environmental stimuli like gravity, distribution of light or the contact with other supports. The gravitropism was highlighted by \citet{darwin1897} as being a major process in the vertical growth control. It allows the plant to position itself in the field of gravity and participates in the strategy of the tree in term of research of light.\\

%La reaction au gravitropisme se fait à travers la formation d'un bois spécial

In the case of angiosperm the reaction to the gravitropism occurs through the differentiation of a tension wood \citep{Archer1986} often accompanied by an eccentric growth. The tension wood is a peculiar wood tissue that is formed in the upper side of leaning trunks and branches which allows the motricity of the ligneous parts of trees. It presents, in flowering plants, a strong trend wood shrinkage at the time of its maturation. When a sector of tension wood is formed, it acts as an internal shroud which puts itself in tension and allows the tree to bend actively. Its formation is regulated by the plant according to the perceptions of the external stimuli as gravitropism \citep{Yoshida2000, Almeras2009a}.\\ %\citep{CNRS2012}

%La couche G en quelque mots

The classic and typical model of tension wood is the development of a gelatinous layer or \gl in the cell wall which comes to add up or to replace the S\down{3} layer and partially or totally the S\down{2} layer \citep{Ruelle2006, Clair2008, Gorshkova2010}. \gl have been studied a lot from Poplar model \citep{Jourez2003, Coutand2007, Clair2011, Yoshinaga2012, Coutand2014}. Poplar model is one of the three \gl morphologies described by \citep{Onaka1956}. In any cases \gl present a very low rate of lignin \citep{Joseleau2004}  that is why \gl is also named cellulosic cell wall. Microfibrils are nearly parallel to fibres axis. \citet{Clair2008} shows that \gl have the properties of a gel with mesopores between 2 and 50 nm. It has been established that it is the chemical and structural properties of this layer which confer the important shrinkage which tends to raise the tree slowly \citep{Trenard1975, Yamamoto2005}.\\

%Mais la couche G n'est pas la seule solution

However in tropical rain forest the \gl is not the main strategy and approximately two thirds of species do not apply it \citep{Norberg1966, Ruelle2006, Clair2006}. \sa tension wood for example does not produce \gl and its tension wood structure is indistinguishable from normal wood with an optical microscope. \citet{Ruelle2006} sets \sa as reference for tree without \gl in tropical rain forest and highlights differences between tension wood and normal wood only for the scale of the cellulose fiber.\\

%Les études portent généralement sur la dichotomy G pas G alors qu'on montre que non 

Studies always present \gl trait as a dichotomy between species with and without it. \citet{Yoshida2000} compared two species with and without \gl . \citet{Clair2006} presented a study on 21 tropical wood and described these species across the tension wood pattern. \citet{Ruelle2006} studied variability of tension wood accross three tropical species as references for trees with \gl , multilayered secondary cell wall or  “normal” tension wood. 

However, early results have shown \gl in tension wood of \sa at the young stage despite \citet{Ruelle2006} hypothesis and well admitted dichotomy. Following these early results we designed an experiment to highlight the presence of \gl in \sa and to explore some lead of explanation. We tested two hyphtothesis: (i) the presence of \gl is a response beginning for a threshold stress improbable in real condition (ii) the ability to set a \gl is a trait visible only on very young stage which disappear when older.\\

In discussion we will talk about about this breakage of the well admitted dichotomy and we will debate about some implication of these results for phylogeny.
